1. Field of the Invention
The present invention relates generally to the field of antennas. More particularly, the present invention relates to antennas that scan in two dimensions. Specifically, a preferred embodiment of the present invention relates to an evanescent coupling millimeter wave (MMW) antenna wherein two dimensional (2-D) scanning is provided via a rotating waveguide assembly and a separately rotating grating disk. The present invention thus relates to an antenna of the type that can be termed rotational scanning.
2. Discussion of the Related Art
Within this application several publications are referenced by arabic numerals within parentheses. Full citations for these, and other, publications may be found at the end of the specification immediately preceding the claims. The disclosures of all these publications in their entireties are hereby expressly incorporated by reference into the present application for the purposes of indicating the background of the present invention and illustrating the state of the art.
Millimeter wave (MMW) imaging can be defined as picture-taking using a longer (compared to light) wavelength portion of the electromagnetic spectrum. In active imaging, the object or the scene is illuminated by an MMW transmitter and the reflected or scattered energy is intercepted by a receiving antenna. In passive imaging, the difference in the thermal radiation from objects filling the imaged scene is perceived by an antenna and an associated detector (radiometer).
MMW imaging, although inferior in resolution to optical imaging, provides imagery that is less susceptible to adverse weather or atmospheric conditions. These advantages make MMW imaging particularly well suited for astronomical and earth sciences applications. Further, MMW imaging has significantly greater penetration if look-through capability is desired, (e.g., in concealed weapon detection or industrial inspection).
A MMW imaging system consists of three major components: an antenna, a receiver and a signal processing unit..sup.(1) The receiver can be either a heterodyne mixer or a low noise amplifier (LNA) with a detector. Recently, significant progress has been made in developing MMW receivers. However, the lack of a fast scanning antenna to form an image continues to present a bottleneck to the development of a cost-effective MMW imaging system.
In radiometric applications, antenna performance is of particular importance. Gain, ohmic losses, and sidelobe levels are parameters that are of major importance. The primary figure of merit for radiometry is the gain, and a secondary factor is the equivalent noise temperature.
Traditional mechanically scanning antennas have the disadvantage of inertia that must be overcome at the beginning and at the end of each scan..sup.(2) For example, conventional dish (parabolic) antennas are bulky and therefore too slow in two-dimensional raster scanning. Electronically scanned antennas, on the other hand, are fast but have a high noise figure and are expensive. For example, electronically steered phased array antennas are, at MMW frequencies, too lossy and prohibitively expensive. Nevertheless, several imaging systems based on heterodyne and direct detection of MMW energy have been developed.
One unsatisfactory previously recognized approach, in an attempt to solve the problems referred to above, involves using a parabolic dish radiometer with two axis scanning to construct an image. In this parabolic dish approach, an operating frequency of 35 GHz was used, resulting in an angular resolution of approximately 1.degree.. Using single sideband detection, an antenna noise temperature of 1000.degree. K was reported. Considerable scan time was required to form the image, (e.g., minutes), the bulk of the time having been needed to move and stabilize the antenna dish. This time factor is the main problem with using a large dish antenna to form an image since long scan times are incompatible with most moving objects.
Another unsatisfactory previously recognized approach, in an attempt to solve the problems referred to above, uses the forward movement of an aircraft to construct an image along the flight path. In this approach only one-dimensional scanning by the antenna is needed because the movement of the aircraft provides a quasi-second-dimensional scan for imaging.
Yet another unsatisfactory previously recognized approach, in an attempt to solve the problems referred to above, uses an MMW analog of an infrared (IR) focal plane array. In this approach, an 8.times.8 element receiver array operates at 94 GHz and utilizes a 63 cm lens to form an image at the focal plane. The reported pixel, (i.e., antenna), noise temperature of 4000.degree. K is much higher than that for a mechanically scanning system. Like the scanning parabolic dish, the focal plane array approach requires very long, (e.g., minutes), integration times. Further, this MMW focal plane array approach does not provide an economically viable solution for MMW imaging. Summarizing recent progress in MMW receiver technology, it can be concluded that what is needed is a single MMW receiver element with a very high temperature resolution (.ltoreq.0.01.degree. K) and an imaging system that utilizes either a single such receiver or a small number of them.
The below-referenced U.S. Patent, and allowed U.S. Patent Application in which the issue fee has been paid, disclose embodiments that are satisfactory for the purposes for which they were intended. The disclosures of the below-referenced prior U.S. Patent, and U.S. Patent Application, in their entireties are hereby expressly incorporated by reference into the present application for purposes including, but not limited to, indicating the background of the present invention and illustrating the state of the art.
U.S. Pat. No. 5,305,123 discloses a light controlled spatial and angular electromagnetic wave modulator. U.S. Ser. No. 08/382,493, filed Feb. 1, 1995, discloses an evanescent coupling antenna and method of utilization thereof.